MnDOT bridge inspectors often have to find out what lies beneath the surface of Minnesota’s rivers. Thanks to new sonar inspection technology, the Bridge Office now has a way to see previously hidden riverbed floors and underwater bridge structures in far better detail than ever before.
Typically, bridge engineers turn to professional divers to provide information about what’s underwater. But diving inspections don’t always deliver precise information about bridge damage, debris and riverbed topography.
Of the 11,183 Minnesota bridges that span waterways, about 585 require underwater inspection.
In recent years, bridge inspectors turned to underwater inspection technologies to identify areas of interest and direct divers who can inspect hands on. In turbid, sediment-heavy conditions with low visibility such as the Mississippi River, non-optical technologies – laser, radar and sonar – offer safe and useful options. Sonar gathers underwater acoustic data into point clouds for imaging two-and three-dimensional models of conditions.
In winter 2014, a vendor demonstrated the use of sonar at the Third Avenue bridge over the Mississippi River in Minneapolis, where a void was previously discovered during a diving inspection. Acoustic investigation of the frozen-over site delivered a three-dimensional image of a scoured cavity of eroded concrete under a pier in conditions unsuitable to diving inspection. Later that year, the Bridge Office purchased its own three-beam sonar unit with funds secured from MnDOT’s research implementation program. MnDOT is the nation’s first state transportation department to use the technology, according to Petra DeWall, waterway engineer.
The new 3D scanning technology provides much better information than divers can. Nicki Bartelt, hydraulic design engineer, said divers can see up to 2 feet in front of them in good rivers, so most of the work is done by feel.
With the new technology, Bartelt and her colleagues receive a 3D image to base decisions on. Diver reports included only narratives and rough sketches.
“It’s like night and day,” said Nicki Bartelt, hydraulic design engineer. “It’s a picture, but it’s more than that. It’s a point cloud. It’s totally scalable. It is real-world elevations and dimensions. It’s like the difference between Google Earth and a paper map.”
Although the information is far better from these scans, federal bridge inspection standards still require hands-on inspection of bridges, including substructures above and below the water’s surface. For now, the scans will be used to augment diver inspections and other purposes.
“We’re looking to augment with scans to make it safer for the divers,” DeWall said.
The research implementation team identified a host of lessons and best practices after purchasing and testing the new equipment.
“This does have a pretty steep learning curve,” Bartelt said. “It’s not something you can just buy and use. You have to learn how to use it.”
Getting different pieces of the equipment set up and able to communicate with each other was the first difficult step. Field testing also identified the need for a dedicated generator to provide a consistent portable power source because of difficulty with batteries.
MnDOT will develop and publish an underwater imaging policy and reach out to districts, counties, cities and other bridge owners to promote its imaging capability. The hydraulics unit will develop data on completed projects, generate a list of bridges that suit underwater imaging and ensure field personnel are trained to use imaging techniques and inspection.
“I think it’s really exciting, because it opens your eyes to what’s going on in the river,” said DeWall. “We always just assumed before, but now we can see it and document it. The fact that you have the ability to rotate the picture and move it around, and zoom through it makes a huge difference.”
These videos explain the sonar inspection technology:
- Central Ave Pier 5 Before and After Repair
- Underwater 3D Scanning at I-90 over Rock River, Luverne
- 3rd Ave. Bridge Project—Minneapolis